“The real concern that I have is that in 2015 we had 986 cases involving minors, yet the official figures show … only 928 actual crime recorded incidents. That means … potentially the cases involving minors are not being investigated properly.

“The fact that the number of crimes recorded hasn’t even reached the number of minors tells me that all these cases … are not being investigated properly because until they are recorded properly … we can’t be satisfied that it is, or it isn’t, a crime of modern slavery.”

The number of potential slavery victims being brought to the UK has risen from under 1,000 in 2011 to more than 3,200 last year, but Hyland said government estimates suggest the real figure could be as high as 13,000.

The commissioner insisted progress was being made in achieving convictions.

“There is some good news. Last year, there were far more convictions and this year, we have already passed the amount of convictions for the whole of last year. So it is going in the right direction,” he said.